1. Field of the Invention
The present invention relates to pressure relief valves, and, more particularly, to a manually operable deflation valve for the controlled release of air from a pressurized system, the valve being particularly suitable for use in conjunction with a blood pressure measuring device or sphygmomanometer.
2. Description of the Prior Art
Various manually operable pressure relief valves, adapted for blood pressure measuring purposes, are already known from the prior art in this field. In a standard auscultatory blood pressure test, as suggested by Riva-Rocci and Korotkoff, an inflatable cuff is attached to the limb of the person to be examined, whereupon the cuff is inflated by means of suitable inflating means, until a pressure is reached which lies safely above a level at which the systolic blood pressure would normally be found. The blood pressure test is performed by slowly deflating the measuring cuff, through the controlled release of air from the measuring apparatus or sphygmomanometer, until characteristic pulsating blood circulation noises (the so-called Korotkoff noises) are audible through a stethoscope which is placed over an artery of the limb, for example. The pressure at which these pulsating noises first appear is the systolic blood pressure. In most cases, this pressure is ascertained visually on a manometer or pressure gauge which is connected to the pressurized system. As the pressure inside the measuring cuff is further reduced, through the further gradual release of air from the system, the pulsating blood circulation noise will disappear at a certain pressure level which is representative of the diastolic blood pressure. Once the systolic and diastolic pressure readings have been determined in this manner, the apparatus is completely deflated.
It is important that the final deflation be performed very rapidly, in what is known as the "dumping" of the remaining air pressure. On the other hand, it is extremely important that the rate of deflation of the measuring cuff be controllable very accurately and in precise response to the manual touch control of the air deflation valve. The latter, accordingly, must have a slow-deflation mode of operation and a dumping mode of operation.
In one prior art device, disclosed in U.S. Pat. No. 2,006,878, this dual-mode operation is achieved by arranging the deflation valve in the form of mating male and female threads which, while engaged over a very short thread length, control the slow-deflation mode, the complete disengagement of the threads initiating the dumping mode. In addition to the structural complexity of its parts, this valve has the disadvantage of requiring more than one turn for its operation, necessitating repeated gripping of the valve knob with two fingers.
Another prior art deflation valve is disclosed in U.S. Pat. No. 3,254,671, featuring a simple spring-preloaded disc valve which, for dumping purposes, has a lockable open position. This valve lacks the required sensitivity of fine adjustment in the slow-deflation mode, as a minimal depression of the valve plunger under finger pressure immediately creates a large valve opening. A similar deflation valve, with similar disadvantages, is disclosed in U.S. Pat. No. 3,738,357. In this device, the valve seat and valve plunger are tapered, so that less finger pressure, but longer finger movement is necessary; also, the accidental movement of the valve into the dumping mode is prevented by a special stop.
Still another deflation valve is disclosed in U.S. Pat. No. 3,779,236, where the valve action is provided by a plunger which, by moving radially inwardly against a tubular rubber sleeve, pushes the latter away from the plunger bore, so that air can escape alongside the plunger. A toggle-type lever, operated by a finger, actuates the valve plunger with a special cam configuration which also makes it possible to hold the plunger in a locked open position, for the dumping mode.
Still another prior art deflation valve is disclosed in U.S. Pat. No. 4,013,265. This device features a long, slightly tapered valve plunger which is axially openable, against a spring, by means of a pivotable valve lever. A dumping mode is obtained, when the valve plunger is moved to a position in which bypass channels in the plunger body are exposed. In order to prevent an inadvertant movement of the plunger into the dumping mode, there is provided a threshold in the spring bias which opposes the plunger movement; a detent action holds the valve open for dumping purposes.
To the extent that the various prior art devices mentioned above offer sufficient sensitivity of adjustment during the slow-deflation mode, they are relatively complex in structure and therefore expensive. Several of these valves require more than one finger for their operation, so that the hand which holds the inflation bulb cannot perform the deflation control maneuvers, without releasing the bulb and thereby necessitating assistance with the other hand.